It would have been worth mentioning the 'copy' module. 'copy.copy' for shallow copies of any object, 'copy.deepcopy' for recursive copies.
>>> a = [1, [2, 3]] >>> b = list(a) # or a[:], they are identical >>> a[1].append(4) >>> a.append(5) >>> a [1, [2, 3, 4], 5] >>> b [1, [2, 3, 4]]
If you had used copy.deepcopy, b would now be [1, [2, 3]] as intended.
of course, what with classes i very rarely have deep lists anyway. in fact, copying lists is seldom an issue i run into (though i always run a few tests at the prompt to make sure my understanding is right because mutability bugs can be hard to track down later).
Of course, even if true, that may not matter.
See, I'm all about the weasel words today.
def copy(x):
"""Shallow copy operation on arbitrary Python objects.
See the module's __doc__ string for more info.
"""
cls = type(x)
copier = _copy_dispatch.get(cls)
if copier:
return copier(x)
# more after this point but it's not relevant for lists